The Work of Wings
Have you noticed the curved shape of a bird's wing? An airplane's wing
is curved also. A wing is designed for flight. It has a special shape
called an airfoil. Airfoil shapes can be found on wings, fans and propellers.
The airfoil shape provides a lifting force when air flows around it. An
airfoil has a thicker; rounded leading edge (front end) and a very thin
trailing edge (or back end). In between the leading and trailing edge
it is curved both on the top and bottom surfaces. The top surface usually
has a greater curve (or hump) than the bottom surface. When a surface
is curved we say it has camber.
Have you noticed the curved shape of a bird's wing? An airplane's wing is curved also. A wing is designed for flight. It has a special shape called an airfoil. Airfoil shapes can be found on wings, fans and propellers. The airfoil shape provides a lifting force when air flows around it. An airfoil has a thicker; rounded leading edge (front end) and a very thin trailing edge (or back end). In between the leading and trailing edge it is curved both on the top and bottom surfaces. The top surface usually has a greater curve (or hump) than the bottom surface. When a surface is curved we say it has camber.
An airfoil takes advantage of Bernoulli's Principle. Since the top surface of the wing has more camber than the bottom surface, the air flows faster over the top of the wing than it does underneath. This means that there is less air pressure above the wing than there is beneath the wing. The difference in air pressure above and below the wing causes lift.
How much lift does a wing make?
The amount of lift depends on these things:
An airfoil shape is used to give the greatest lift possible to an airplane. A flat plate held at the proper angle of attack does generate lift, but also generates a lot of drag. Sir George Cayley and Otto Lilienthal during the 1800's showed that curved surfaces generate more lift and less drag than flat surfaces. Early research also showed that a round leading edge and a sharp, flat trailing edge add to a wing's ability to generate more lift and less drag.
Let's construct step-by-step an airfoil section.
A. The length of the airfoil section is determined by placing the leading and trailing edges their desired distance apart. This length is called the chord line.
B. Add curvature with the camber line. The amount of curvature is determined by the camber line. This curvature greatly helps generate lift.
C. Add thickness above the camber line. The amount of thickness that is added will depend on the amount of strength needed in the wing and the speed the airplane will usually fly.
D. Add the same amount of thickness below the camber line.
E. Now you have an airfoil shape.
Different airfoil shapes generate different amounts of lift and drag. If an airplane is being designed to fly at low speed (0 - 100 mph), it will have a different airfoil shape than an airplane designed to fly at supersonic speed (760 - 3,500 mph). That's because the air flows in slightly different ways at different speeds and at different altitudes. In general, low to medium speed airplanes have airfoils with more thickness and camber.
Because the airplane is not moving through the air very fast the wing needs to generate as much lift as possible at a slower speed. The air density at lower altitudes is greater. More molecules in the air generate more lift than fewer molecules in the same amount of air. Greater camber gives greater lift at slower speeds. At faster speeds (supersonic) and at higher altitudes airfoil shapes need to be thinner. That's because when flying close to or at the speed of sound a shock wave forms at the nose of the airplane. NASA researchers discovered that a thin airfoil delays the formation of the shock wave. This reduces drag that is caused as the airplane moves through the shock wave.
During the 1940's, the National Advisory Committee for Aeronautics (NACA) did research on different airfoil shapes. Their investigations gave results that are still used today to influence the design of new aircraft.
When engineers design a new airplane, the size and shape of the wings are very important to efficient flight. Wings provide the majority of the lift for an airplane, but wings also cause drag. Air flowing over the top of a wing also tends to flow inward toward the fuselage. Meanwhile the air flowing underneath the wing tends to flow outward. As these two airflows meet along the trailing edge of the wing, they form a rotating column of air that extends from the wingtips. This is called a wingtip vortex. These are visible from a passenger's seat next to the wing on humid days, cold, moist mornings or flying through mist.
Energy is lost in the process of making lift because of the airflow around the wingtips. A wingtip vortex generates a lot of drag. If the lift is spread out over a longer wingspan, the effects of the wingtip vortex are not as great. Engineers have found that designing wings with greater aspect ratio lessens that drag. Aspect ratio is a comparison between the length and width of a wing.
Let's do the math. We will keep all other things about the wing the same (weight, airfoil shape, material that the wing is made out of, speed at which the airplane is designed to fly, things like that). We first measure the wing's length and width. We then perform our calculation: length divided by width. Our answer (the quotient) will imply just how great the wingtip drag will be for this wing. The greater the number for aspect ratio, the less the wingtip drag.
Let's look at an example. Take two wings with the same amount of area (let's say 100 square units), but with different lengths and widths.
Now figure the aspect ratio for each wing. The wing with the greater quotient will have less wingtip drag.
Experiments have shown that a wing built with a greater aspect ratio tends to create less drag than a wing built with a lesser aspect ratio even when their area remains the same.
Long slender wings like those on a sailplane are called "high aspect ratio" wings, and are much more efficient at making lift without very much drag. Low aspect ratio wings like on a fighter airplane have much more of this type of drag.
The shape of a wing greatly influences the performance of an airplane. The speed of an airplane, its maneuverability, its handling qualities, all are very dependent on the shape of the wings. There are, for our purposes here, 3 basic wing types that are used on modern airplanes: straight, sweep and delta.
The straight wing is found mostly on small, low-speed airplanes. General Aviation airplanes often have straight wings. Sailplanes also use a straight wing design. These wings give the most efficient lift at low speeds, but are not very good for high speed flight approaching the speed of sound.
The swept wing (forward swept or sweptback) is the wing design of choice for most modern high speed airplanes. The swept wing design creates less drag, but is somewhat more unstable for flight at low speeds. A high sweep wing delays the formation of shock waves on the airplane as it nears the speed of sound. How much sweep a wing design is given depends upon the purpose for which the airplane is designed to be used. A commercial jetliner has a moderate sweep. This results in less drag, while maintaining stability at lower speeds. High speed airplanes (like modern jet fighters) have a greater sweep. These airplanes do not generate much lift very during low speed flight. Airplanes with sweep need to take off and land at high speeds.
From above, a delta wing looks like a large triangle. It has a high sweep with a straight, trailing edge. Because of this high sweep, airplanes with this wing are designed to reach supersonic speeds. The landing speed of these delta-winged aircraft is also fairly fast. This wing shape is found on the supersonic transport Concorde and the Space Shuttles.
As long as the airflow can move smoothly over and under the wing, the lift will increase along with the angle of attack. At a certain point though, the angle of attack is so great that the smooth (or attached) airflow cannot follow the shape on the upper side of the wing. The airflow will then stop following the shape of the wing. The airflow will spread out and away from the wing's surface. This is called airflow separation.
Every wing has a particular angle of attack for certain speeds at which the airflow separates from the wing's surface. This point is called the stall angle. When an airplane's wing reaches the stall angle, the wing stops generating lift. (Exploring Aero animation from Lift segment)
Air density is measured by how tightly compressed the molecules are. Air molecules in the lower layers of the atmosphere are closer together than the air molecules in the upper atmosphere. When there are more molecules in the air (greater density), it is easier to generate lift. Fewer molecules in the air make it more difficult to generate lift. That's why it is easier to fly airplanes in the layer of atmosphere closest to the Earth's surface. There are more molecules closest to the Earth's surface.
There is another kind of drag that has to do with compressing air molecules in the atmosphere. When flying close to the speed of sound or at the speed of sound (Mach 1), the airflow around an aircraft acts differently than at slower speeds. As the aircraft moves through the air it makes pressure waves. These pressure waves stream out away from the aircraft at the speed of sound. This wave acts just like the ripples through water after a stone is dropped in the middle of a still pond. At Mach 1 or during transonic speed (Mach 0.7 - 0.9), the aircraft actually catches up with its own pressure waves. These pressure waves turn into one big shock wave. It is this shock wave that buffets the airplane. The shock wave also creates high drag on the airplane and slows the airplane's speed. As the airplane passes through the shock wave it is moving faster than the sound it makes. The shock wave forms an invisible cone of sound that stretches out toward the ground. When the shock wave hits the ground it causes a sonic boom that sounds like a loud thunderclap.
The energy lost in the process of compressing the airflow through these shock waves is called wave drag. This reduces lift on the airplane.
Table of Contents